Automatic lengthwise receivers presently known, for example, such as described in French Pat. No. 2,357,456 by the applicants, employ between the output from the feeder machine and the stack formation station a system of at least two continuous conveyors, a first conveyor receiving at its upstream end the panels leaving the feeder machine and depositing them at its downstream end onto a second conveyor which in turn deposits the panels at the stack formation station, which generally comprises an elevator table having motor-driven rollers. Furthermore such receivers are equipped with means enabling a change of stack to be carried out, these means enabling a stop to be interposed at the output from the second conveyor when a stack is finished, the speed of this second conveyor to be reduced or brought to zero, the finished stack to be discharged, and finally the stop to be retracted and normal speed of the second conveyor to be reestablished when the stacking station is ready to receive the next stack. Furthermore numerous devices of this type are such that the first conveyor includes a telescopic portion downstream, and are equipped with means enabling this telescopic portion to be shifted so that the point of deposit of the first conveyor shifts between two extreme positions always located above the second conveyor; these devices are advantageously in addition equipped with means of causing retraction of the telescopic portion during stack changes, as well as running it out again progressively, starting from the resumption of the formation of the stack.
As employed at present these devices do not enable the problem of the continuous changing of form and size of the panels to be solved in a satisfactory manner, that is to say, without stopping the corrugator. The intermediate stack formed at the end of the output of panels of a first form and size and at the start of the output of panels of a second form and size includes generally panels of both types, which must then be sorted out manually.